Certain types of fire detecting apparatus utilize an optical detector comprising a pair of photo-cells connected in series across a voltage source. The cells are responsive to different wave lengths of light, so that on receiving sunlight or incandescent light, both cells respond by a drop in resistance, so that the voltage at the junction between the cells changes only slightly. However, when exposed to light with substantially only infrared or red components, the resistance of one cell drops substantially, and the resistance of the other cell drops little, of at all. Hence, assuming that the positive side of the source is connected to the red-responsive cell, the voltage at the junction rises appreciably, and this rise in voltage is used as the input to an amplifier to actuate an alarm. A detector of this type is shown in U.S. Pat. No. 3,188,593 issued June 8, l965.
One problem associated with detector systems of this type is the fact that when a break occurs in the ground lead to the remote optical detectors, the voltage at the junction will rise to nearly the full supply voltage, causing a false alarm. When the detector is connected to automatic extinguishing equipment, a break in the ground line will cause a false firing of the extinguishing medium.